cat-file resource pool

Avoid running a large number of git cat-file child processes when run with
a large -J value.

This implementation takes care to avoid adding any overhead to git-annex
when run without -J. When run with -J, there is a small bit of added
overhead, to manipulate the resource pool. That optimisation added a
fair bit of complexity.

CmdLine/Action.hs

@@ -199,8 +199,7 @@ startConcurrency :: UsedStages -> Annex a -> Annex a
 startConcurrency usedstages a = do
 	fromcmdline <- Annex.getState Annex.concurrency
 	fromgitcfg <- annexJobs <$> Annex.getGitConfig
-	let usegitcfg = Annex.changeState $ 
-		\c -> c { Annex.concurrency = fromgitcfg }
+	let usegitcfg = setConcurrency fromgitcfg
 	case (fromcmdline, fromgitcfg) of
 		(NonConcurrent, NonConcurrent) -> a
 		(Concurrent n, _) ->

CmdLine/GitAnnex/Options.hs

@@ -36,6 +36,7 @@ import CmdLine.GlobalSetter
 import qualified Backend
 import qualified Types.Backend as Backend
 import Utility.HumanTime
+import Annex.Concurrent
 
 -- Global options that are accepted by all git-annex sub-commands,
 -- although not always used.
@@ -395,7 +396,7 @@ jsonProgressOption =
 -- action in `allowConcurrentOutput`.
 jobsOption :: [GlobalOption]
 jobsOption = 
-	[ globalSetter set $ 
+	[ globalSetter setConcurrency $ 
 		option (maybeReader parseConcurrency)
 			( long "jobs" <> short 'J' 
 			<> metavar (paramNumber `paramOr` "cpus")
@@ -403,8 +404,6 @@ jobsOption =
 			<> hidden
 			)
 	]
-  where
-	set v = Annex.changeState $ \s -> s { Annex.concurrency = v }
 
 timeLimitOption :: [GlobalOption]
 timeLimitOption =